Scanning electron micrograph of a larva of Trialeurodes vaporariorum, the glasshouse whitefly, on the underside of a tomato leaf. The picture is of a larva (also called a 'nymph') 0.7mm long, just before a new adult fly emerges. It appears as a sealed elliptical box, with fragile surface tubules of wax (broken fragments visible). The only sign of the contents is at the red region, an operculum. There are four larval stages, or instars; the first has legs, and moves around the leaf surface before settling to feed on sap in the plant's phloem. The legs then degenerate. Feeding continues until the fourth instar (here); the insect develops its final form within the enclosing skin, which then splits to free it. T. vaporariorum is a common nuisance in glasshouses, difficult to control with insecticides. Some whitefly species are serious pests, as vectors for plant virus diseases.